fix(bigquery): handle API-wrapped null values in NULLABLE fields

bigquery/gcloud/aio/bigquery/utils.py

@@ -101,26 +101,28 @@ def parse(field: dict[str, Any], value: Any) -> Any:
         )
         raise
 
+    value = flatten(value)
+
     if field['mode'] == 'NULLABLE' and value is None:
-        return value
+        return None
 
     if field['mode'] == 'REPEATED':
         if field['type'] == 'RECORD':
             return [{
                 f['name']: parse(f, x)
                 for f, x in zip(field['fields'], xs)
             }
-                for xs in flatten(value)]
+                for xs in value]
 
-        return [convert(x) for x in flatten(value)]
+        return [convert(x) for x in value]
 
     if field['type'] == 'RECORD':
         return {
             f['name']: parse(f, x)
-            for f, x in zip(field['fields'], flatten(value))
+            for f, x in zip(field['fields'], value)
         }
 
-    return convert(flatten(value))
+    return convert(value)
 
 
 def query_response_to_dict(response: dict[str, Any]) -> list[dict[str, Any]]:

bigquery/tests/unit/utils_test.py

@@ -105,6 +105,8 @@ def test_parse_nullable(kind):
     # make sure we never convert to a falsey typed equivalent
     # eg. for BOOLEAN, None != False
     assert parse(field, None) is None
+    # BigQuery API wraps null values as {'v': None} -- must not crash
+    assert parse(field, {'v': None}) is None
 
 
 @pytest.mark.parametrize(